Abstract
Despite the wide knowledge about prevalent effects of ocean acidification on single species, the consequences on species interactions that may promote or prevent habitat shifts are still poorly understood. Using natural CO2 vents, we investigated changes in a key tri-trophic chain embedded within all its natural complexity in seagrass systems. We found that seagrass habitats remain stable at vents despite the changes in their tri-trophic components. Under high pCO2, the feeding of a key herbivore (sea urchin) on a less palatable seagrass and its associated epiphytes decreased, whereas the feeding on higher-palatable green algae increased. We also observed a doubled density of a predatory wrasse under acidified conditions. Bottom-up CO2 effects interact with top-down control by predators to maintain the abundance of sea urchin populations under ambient and acidified conditions. The weakened urchin herbivory on a seagrass that was subjected to an intense fish herbivory at vents compensates the overall herbivory pressure on the habitat-forming seagrass. Overall plasticity of the studied system components may contribute to prevent habitat loss and to stabilize the system under acidified conditions. Thus, preserving the network of species interactions in seagrass ecosystems may help to minimize the impacts of ocean acidification in near-future oceans.
Highlights
Despite the wide knowledge about prevalent effects of ocean acidification on single species, the consequences on species interactions that may promote or prevent habitat shifts are still poorly understood
We detected no differences between sites in the niche width or in the number of carbon resources exploited by sea urchin populations, which showed similar Standard Ellipse Areas and bootstrapped Layman metrics (Fig. 1a)
No difference in sea urchin stoichiometry was found between sites (Fig. 2a; PERMANOVA non-significant p-values for CNP contents and ratios)
Summary
Despite the wide knowledge about prevalent effects of ocean acidification on single species, the consequences on species interactions that may promote or prevent habitat shifts are still poorly understood. Some experiments[8] and observations at CO2 vents[12] indicate that under acidified conditions the enhanced resource effect of high pCO2 on turf algae, combined with a weakened top-down control by herbivores (reduced abundance and/or feeding), drive the reorganization of kelp forests towards turf-dominated systems. We studied multiple basal resources, one of the two main seagrass herbivores (sea urchin), and a territorial labrid fish that is known to predate on such herbivore[17,18] (Symphodus tinca, commonly known as peacock wrasse) Both consumers have restricted benthic home ranges, which ensures long-term exposure to high pCO2 levels at the vent sites for them and their resources. Herbivore abundance, resulting from the propagation of both, resource- and predator-driven effects, was quantified
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